Method and apparatus for sending feedback for a downlink shared service and estimating a number of wireless transmit/receive units

ABSTRACT

A method and an apparatus for sending a signal on a contentious feedback channel for a downlink shared service and for estimating a number of wireless transmit/receive units (WTRUs) are disclosed. When a transmission criterion associated with a physical random access feedback channel (P-RAFCH) is satisfied, a physical resource is randomly selected among a plurality of physical resources assigned for the P-RAFCH and a pre-configured signal is sent using the selected physical resource. A Node-B receives the pre-configured signal from a plurality of WTRUs and calculates a number of the WTRUs based on a number of used physical resources. The transmission criterion is at least one of successful reception of a data packet on a downlink physical channel, successful reception of a data block on a data service, reception of a signaling command, occurrence of a measurement event, and failure to receive a transmission after a specified number of times.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/869,923 filed Oct. 10, 2007, which claims the benefit ofU.S. provisional application Ser. Nos. 60/828,881 filed Oct. 10, 2006and 60/883,594 filed Jan. 5, 2007, which are incorporated by referenceas if fully set forth. This application also claims the benefit of U.S.provisional application Ser. No. 61/093,142 filed Aug. 29, 2008, whichis incorporated by reference as if fully set forth.

FIELD OF INVENTION

The present invention is related to wireless communications.

BACKGROUND

Introduction of downlink shared services, (i.e., broadcast or multicasttransmissions), over a high speed physical downlink shared channel(HS-PDSCH) has been discussed in several contexts including enhancedmultimedia broadcast multicast services (MBMS) and transmissions towireless transmit/receive units (WTRUs) in a radio resource control(RRC) CELL_FACH state. For the downlink shared services, the same datastream is intended for a plurality of WTRUs that are known or thought tobe in a cell, and the network may allow the data to be viewable to otherWTRUs. Guarantee of data delivery to some or most of the WTRUs isimportant and a mechanism to provide such a guarantee should besupported.

Using HS-PDSCH or similar channel for delivery of the downlink sharedservices offers several advantages. The HS-PDSCH is a shared physicalchannel well suited for delivery of services across a wide-range ofquality of service (QoS) classes. The HS-PDSCH is also optimized forpacket services as most shared services are likely to be, (e.g., forwardaccess channel (FACH) data and MBMS data are most likely packetized).The HS-PDSCH also supports hybrid automatic repeat request (HARQ), whichcan be used to guarantee or significantly improve packet delivery.

In order to take advantage of the HARQ mechanism of HS-PDSCH, a feedbackmechanism is required which allows the WTRUs to send a positiveacknowledgement (ACK) or a negative acknowledgement (NACK) to a Node-B.In high speed downlink packet access (HSDPA), the ACK or NACK message isdelivered to the Node-B via a dedicated uplink channel, (i.e., highspeed dedicated physical control channel (HS-DPCCH)). This not onlyguarantees availability of channel resources to deliver the ACK or NACKmessage, it also allows the Node-B to identify which WTRU a particularACK or NACK message originates from.

Additionally, performance of HSDPA is significantly enhanced through theavailability of channel quality indicator (CQI) feedback from the WTRUs.Conventionally, the CQI is also sent via the HS-DPCCH and the Node-B mayidentify the source of the CQI.

While the approach above is practical when the HS-PDSCH is primarilyused to carry dedicated data in a CELL_DCH state, it is no longerpractical for delivery of shared data or dedicated data when the WTRUsare operating in a CELL_FACH state. Any other currently availablemechanisms for delivery of ACK/NACK and CQI feedback are insufficientfor operation outside of CELL_DCH state, (i.e., when dedicated resourcesare not available). There may be a very large number of WTRUs listeningto a particular shared service in a cell. Dedicating a resource to theseWTRUs and requiring ACK/NACK feedback of every single packet from theseWTRUs will have a highly detrimental impact on the uplink capacity ofthe communication systems. Moreover, WTRUs not registered in a cellcannot have an access to the resources.

Since a dedicated resource is not allocated in a CELL_FACH state, theonly currently available alternative for delivering an ACK or NACKmessage and a CQI is via a random access channel (RACH). Delivering anACK or NACK message and a CQI via a RACH would likely to have a severeimpact on the uplink capacity and is not practical. If the ACK or NACKmessages and a CQI are delivered from all WTRUs, given that the downlinkdata is shared among a large number of WTRUs, conventional RACHoperation may require a large number of retransmission of almost alldata. Therefore, delivering feedback via a conventional RACH isimpractical.

It would be desirable to provide a mechanism for feedback from WTRUs fora downlink shared service, while the impact on the uplink and downlinkcapacity is minimal.

SUMMARY

A method and an apparatus for sending a signal on a contentious feedbackchannel for a downlink shared service and for estimating a number ofWTRUs are disclosed. When a transmission criterion associated with aphysical random access feedback channel (P-RAFCH) is satisfied, aphysical resource is randomly selected among a plurality of physicalresources assigned for the P-RAFCH and a pre-configured signal is sentusing the selected physical resource. A Node-B receives thepre-configured signal from a plurality of WTRUs and calculates a numberof the WTRUs based on a number of used physical resources. Thetransmission criterion is at least one of successful reception of a datapacket on a downlink physical channel, successful reception of a datablock on a data service, reception of a signaling command, occurrence ofa measurement event, or failure to receive a transmission after aspecified number of times.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding of the invention may be had from thefollowing description, given by way of example and to be understood inconjunction with the accompanying drawings wherein:

FIG. 1 is a block diagram of an example WTRU in accordance with oneembodiment;

FIG. 2 is a block diagram of an example Node-B in accordance with oneembodiment;

FIG. 3 is a flow diagram of a process for providing feedback for adownlink shared service via a downlink shared channel in accordance withone embodiment;

FIG. 4 shows one possible power variation scheme of an HS-PDSCH;

FIG. 5 is a flow diagram of a process for providing feedback for adownlink shared service transmitted to a plurality of WTRUs via HSDPA inaccordance with another embodiment; and

FIG. 6 is a flow diagram of an example process of evaluating atransmission criterion for transmitting a P-RAFCH.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

When referred to hereafter, the terminology “WTRU” includes but is notlimited to a user equipment (UE), a mobile station, a fixed or mobilesubscriber unit, a pager, a cellular telephone, a personal digitalassistant (PDA), a computer, or any other type of user device capable ofoperating in a wireless environment. When referred to hereafter, theterminology “Node-B” includes but is not limited to a base station, asite controller, an access point (AP), or any other type of interfacingdevice capable of operating in a wireless environment.

FIG. 1 shows a wireless communication system 10 including a plurality ofWTRUs 100, a Node-B 120, a controlling radio network controller (CRNC)130, a serving radio network controller (SRNC) 140, and a core network150. The Node-B 120 and the CRNC 130 may collectively be referred to asuniversal terrestrial radio access network (UTRAN).

As shown in FIG. 1, the WTRUs 100 are in communication with the Node-B120, which is in communication with the CRNC 130 and the SRNC 140.Although three WTRUs 100, one Node-B 120, one CRNC 130, and one SRNC 140are shown in FIG. 1, it should be noted that any combination of wirelessand wired devices may be included in the wireless communication system100.

FIG. 2 is a functional block diagram of a WTRU 100 and the Node-B 120 ofthe wireless communication system 10 of FIG. 1. As shown in FIG. 2, theWTRU 100 is in communication with the Node-B 120 and both are configuredto perform a method of sending feedback for a downlink shared servicedand estimating a number of WTRUs in a cell.

In addition to the components that may be found in a typical WTRU, theWTRU 100 includes a transmitter 102, a receiver 104, a decoder 106, aCQI measurement unit 108 (optional), a memory 110, a controller 112, andan antenna 114. The memory 110 is provided to store software includingoperating system, application, etc. The controller 112 is configured toperform a method of sending feedback for a downlink shared service. Thereceiver 104 and the transmitter 102 are in communication with thecontroller 112. The antenna 114 is in communication with both thetransmitter 102 and the receiver 104 to facilitate the transmission andreception of wireless data.

The receiver 104 receives signals from a Node-B. The decoder 106 decodesthe received signal from the Node-B. The decoder 106 may decode a highspeed shared control channel (HS-SCCH) signal while the WTRU 100 is in aCell_FACH state. The decoder 106 may decode a downlink transmission on ahigh speed physical downlink shared channel (HS-PDSCH) if the WTRU 100successfully decodes an identity (ID) of the WTRU 100 on the signal onthe HS-SCCH. The transmitter 102 sends feedback, (i.e., a CQI or anacknowledgement based on the decoding of the downlink transmission), toa Node-B via a contention-based shared feedback channel, which will beexplained in detail below. The CQI measurement unit 108 outputs a CQI,which will be explained in detail below.

In addition to the components that may be found in a typical Node-B, theNode-B 120 includes a encoder, 202, a transmitter 204, a receiver 206, acontroller 208, and an antenna 210. The controller 208 is configured toperform a method of estimating a number of WTRUs in a cell. Thetransmitter 202 and the receiver 204 are in communication with thecontroller 208. The antenna 210 is in communication with both thetransmitter 202 and the receiver 204 to facilitate the transmission andreception of wireless data.

The encoder 202 encodes data stream(s) for transmission. The transmitter204 sends a downlink transmission including the encoded data stream(s)for a downlink shared service to a plurality of WTRUs via a downlinkshared channel. The controller 208 controls a downlink transmit powerand/or an MCS on the downlink shared channel so that the downlinktransmissions are transmitted to the WTRUs with a high likelihood ofsuccess of being received. The receiver 206 receives feedback from theWTRUs via a contention-based shared feedback channel.

FIG. 3 is a flow diagram of a process 300 for providing feedback for adownlink shared service via a downlink shared channel in accordance withone embodiment. A WTRU 100 receives a downlink transmission via adownlink shared channel for a downlink shared service that is providedto a plurality of WTRUs from a Node-B 120 (step 302). The WTRU 100decodes the downlink transmission (step 304). If the decoding is notsuccessful, the WTRU 100 sends a pre-defined burst signifying a negativeacknowledgement (NACK) to the Node-B 120 via a contention-based sharedfeedback channel (step 306). The pre-defined burst may be sent only oncewithout requiring an acknowledgement from the Node-B 120. If thedecoding is successful, the WTRU 100 does not send feedback, (i.e., anACK is implicit).

A new uplink shared feedback channel, a physical random access feedbackchannel (P-RAFCH), is introduced for sending the feedback from the WTRU100 to the Node-B 120. The P-RAFCH is a contention-based random accesschannel. At least one P-RAFCH may be associated with each HS-SCCH in thedownlink. If several downlink shared services are supported over theHS-PDSCH(s), a set of P-RAFCHs are provided for the downlink sharedservices and each P-RAFCH may be dedicated to a particular downlinkshared service.

The configuration of the shared feedback channel, (i.e., P-RAFCH), maybe communicated via system information block (SIB) and may varycell-by-cell. Alternatively, the shared feedback channel configurationmay be signaled through dedicated RRC signaling to the WTRUs that have aconnection to the radio access network (RAN), (e.g., WTRUs operating ina CELL_FACH state). The Node-B 120 broadcasts available scrambling codesand access slots for the shared feedback channel. The access slotduration may be the same as for the conventional RACH, and may bematched, (i.e., derived), to the transmission time interval (TTI) of thedownlink shared services. When a WTRU 100 needs to provide feedback, theWTRU 100 randomly selects a physical resource, (e.g., a code and anaccess slot), associated with a particular TTI on a particular downlinkshared service and sends its feedback.

It should be noted that the P-RAFCH may be defined by a combination ofany physical resources including, but not limited to, code, subcarrier,time, space, etc., and the exact definition of the P-RAFCH physicalresource is not essential to the embodiments disclosed herein.

In transmission of the feedback, (i.e., the pre-defined burst), notransmit power ramp-up mechanism is used in contrast to the conventionalRACH. The WTRU 100 may send each feedback only once and does not requireacknowledgement of its receipt from the Node-B 120. The transmit powerfor the feedback may be determined based on the received power measuredon a reference channel, (e.g., common pilot channel (CPICH), HS-PDSCH,etc.), and a network-supplied offset. The offset value may be includedin an SIB. Alternatively, the network may instruct the WTRU 100 to usean absolute power, and provides a rule when the WTRU 100 is allowed toprovide feedback. For example, the WTRU 100 may be permitted to sendfeedback only if the received reference channel power is above apre-defined value.

If the WTRU 100 has selected a Node-B out of several synchronizedNode-Bs which transmit the same downlink transmission, the WTRU 100transmits a NACK only to that selected Node-B. If the WTRU 100 performssoft combining of signals from a plurality of Node-Bs in an active set,the WTRU 100 sends a NACK to the strongest Node-B in the active set.

The WTRU 100 may send a NACK each time the WTRU 100 fails to decode thedownlink transmission. Alternatively, the WTRU 100 may send a NACK aftertwo or more successive downlink transmissions have failed. For example,the WTRU 100 may send a NACK only if m out of n successive transmissionshave failed. The numbers m and n may be determined by the network. Forthe purpose of counting m out of n, original transmissions,re-transmissions, both, or relative combination of both may be counted.The ability to actually send the NACK may depend on some random numberwith probability set by the network. The network may indicate desiredtransmission of the NACK on a cell different from the one where thedownlink shared service, (e.g., MBMS), is received. The cell isindicated by the network.

In one embodiment, the feedback may be anonymous. If the feedback goesthrough, the Node-B 120 knows that some WTRU in the cell was not able todecode the downlink transmission in a particular TTI. Alternatively, theWTRU ID may be signaled. In accordance with one embodiment, the downlinkshared service may be mapped to a WTRU-specific signature code that willbe transmitted as the payload of the P-RAFCH. In accordance with anotherembodiment, a WTRU connection ID may be signaled along with thefeedback. In accordance with yet another embodiment, the accessopportunities to the contention-based shared feedback channel may bemapped to the downlink shared service so that the WTRU ID may beverified based on the predefined mapping. The mapping may be transmittedby the network.

The Node-B 120 calibrates the transmit power and/or adjusts an MCS ofthe downlink shared channel carrying the shared downlink service so thatit covers the desired coverage area, (i.e., cell or a sector of a cell),with a high likelihood. With the transmit power and/or MCS adjustment,the probability that a WTRU 100 will not receive the downlink data in aTTI can be set to a desired operating point, preferably near zero. Sincea WTRU 100 sending a NACK is almost certainly at the edge of the cell orsector, the downlink power computation should be done under thisassumption. Since the Node-B 120 knows the cell or sector size, theNode-B 120 may configure the downlink transmit power and/or MCS so thatit does not significantly interfere with other signals. Consequently,only very few WTRUs may need to send a NACK for any single TTI. Withthis approach where feedback power is fixed, a rule may be set toprohibit WTRUs from sending feedback.

Since a WTRU 100 sending a NACK is almost certainly at the edge of thecell or sector, the uplink transmit power on the shared feedbackchannel, (e.g., P-RAFCH), may be determined under this assumption. Sincethe Node-B 120 knows the cell or sector size, the Node-B 120 configuresthe uplink transmit power such that it does not significantly interferewith other signals at the Node-B 120.

Under the above assumption, (very few NACKs expected per TTI), theNode-B 120 may allocate enough shared feedback channel resources so thatthe probability of collision for a NACK is kept low and the Node-B 120is able to receive a large number of NACKs without severely impactingthe uplink capacity.

If the Node-B 120 receives at least one NACK, the Node-B 120 schedules aretransmission for which the NACK is received. In this way, the HS-PDSCHoperates as it conventionally does under normal HSDPA operation. Packetdelivery is guaranteed to the same extent as it is guaranteed under thecurrent HARQ, (i.e., subject to a maximum limit on re-transmissions anderrors in the feedback of NACKs).

The Node-B may maintain a threshold value and retransmit the downlinktransmission only if the number of NACKs from the WTRUs exceeds thethreshold value. While data delivery is not guaranteed, it is guaranteedthat no more than a few WTRUs are unhappy. This limits the impact on thedownlink shared service throughput of a small number of WTRUs.Alternatively, the Node-B 120 may ignore the NACKs. The Node-B 120 mayallocate no resources to the shared feedback channel to obtain the sameresult.

The Node-B 120 may pool the NACKs, (i.e., keep track of data that needsretransmission), and retransmit multiple downlink transmissions at alater time as a single packet. In this case, a sequence number andbuffering may need to be extended.

The Node-B 120 may implement the following downlink power controlmechanism for the HS-PDSCH. Let P_(n) be the HS-PDSCH power reference,(i.e., power per bit), for TTI n. If a NACK is received, the Node-B 120may set the transmit power reference for TTI (n+1) as follows:

P _(n+1) =P _(n)+ƒ(num.of NACKs)Δ_(NACK); or  Equation (1)

P _(n+1) =P _(MAX).  Equation (2)

If the Node-B 120 receives no NACKs, the Node-B 120 may set the transmitpower reference for TTI (n+1) as follows:

P _(n+1) =P _(n)−Δ_(ACK).  Equation (3)

Here, Δ_(ACK),Δ_(NACK)>0, ƒ( ) is a positive non-decreasing (but may beconstant) function of its argument. If the Node-B 120 does not receiveany NACKs, the Node-B 120 may bring the transmit power reference down bya pre-defined decrement. As soon as a NACK is received, the transmitpower reference may be increased by a pre-defined increment. Thepre-defined increment and decrement may or may not be the same. Theincrease may depend on the number of NACKs received (but maybeconstant). The increment ƒ(num.of NACKs)Δ_(NACK) is preferably muchlarger than the decrement Δ_(ACK). FIG. 4 shows one possible powervariation scheme of an HS-PDSCH.

The actual transmit power in TTI n depends on P_(n) and the data formatselected for the data, as it does conventionally. Additionally, amaximum and a minimum power may be set to limit the actual transmitpower.

In addition to, or as an alternative to, the transmit power control, theNode-B 120 may adjust an MCS of the downlink shared service in a similarfashion. When no NACK is received, the Node-B 120 may increase the MCSorder, and when at least one NACK is received, the Node-B 120 may lowerthe MCS order.

For both power control and MCS control, the Node-B 120 may consider theresources allocated to other services in determining the range ofpossible transmit power and MCS. For instance, if the load created byother services is low, the Node-B 120 may increase its transmissionpower and/or reduce the MCS utilized for the downlink shared services,which allows more WTRUs to decode the service.

When the Node-B 120 needs to know how many WTRUs are listening to thedownlink shared service, the Node-B 120 may temporarily, (e.g., one (1)TTI), request all WTRUs to send NACKs. For this, the Node-B 120 may senda special burst or a data sequence with intentionally erroneous CRCcheck. This will force all WTRUs to respond with a NACK. The Node-B 120counts the number of received NACKs, making allowances for losses due tofading and collisions. Not only does this provide a count that should beapproximately correct, but if the NACK power is “absolute”, (as opposedto relative to a received power), a distribution of uplink channelqualities is also obtained.

FIG. 5 is a flow diagram of an example process 500 for providingfeedback for downlink shared services to WTRUs via HSDPA in accordancewith another embodiment. A WTRU 100 receives signaling on an HS-SCCHfrom a Node-B 120 while the WTRU is in a Cell_FACH state (step 502). TheWTRU 100 decodes a downlink transmission on an HS-PDSCH if the WTRU 100successfully decodes an identity of the WTRU 100 on the signaling on theHS-SCCH (step 504). The WTRU 100 sends an acknowledgement to the Node-B120 based on the decoding of the downlink transmission via acontention-based shared feedback channel (step 506). The transmission onthe shared feedback channel and the signaling on the HS-SCCH have afixed timing relationship.

One shared feedback channel may comprise one scrambling code and onechannelization code, (or alternatively any combination of physicalresources), in the uplink. At least one shared feedback channel isassociated with each HS-SCCH in the downlink. The shared feedbackchannel is shared amongst all WTRUs in a CELL_FACH that are requested tomonitor the associated HS-SCCH.

Transmission over the shared feedback channel by different WTRUs may betime multiplexed, and follow a timing restriction with respect to thesignaling over the HS-SCCH. More specifically, a WTRU 100 transmits anACK or NACK message over the associated shared feedback channel at afixed time interval after having successfully decoded its WTRU ID,(i.e., high speed radio network temporary identity (H-RNTI)) over theHS-SCCH. The duration of the time interval should be set such that it islong enough for the WTRU 100 to receive and decode the data on theHS-PDSCH and evaluate whether there was an error, (i.e., cyclicredundancy check (CRC) verification), yet short enough to allow theNode-B 120 to quickly retransmit an erroneous transport block as part ofthe HARQ processing. The transmission over the shared feedback channelmust last at most one (1) TTI length to avoid collisions between WTRUstransmitting feedback. Moreover, an adequate guard period should bedefined to avoid WTRUs with different timing offsets, (e.g., near-farproblem), from colliding when transmitting over the shared feedbackchannel.

The information and parameters related to the shared feedback channelmay be signaled to the WTRU 100 at the same time as HS-SCCH-relatedinformation is signaled, either through an SIB over the broadcastcontrol channel (BCCH)/broadcast channel (BCH) or through dedicated RRCsignaling, (e.g., new information element (IE) in the RRC CONNECTIONSETUP message).

The transmission power at which a WTRU 100 sends the feedback may be setbased on the received power measured on a reference channel, (e.g.,CPICH, HS-PDSCH, etc.), and a network-supplied offset value. The offsetvalue may be part of the SIB. Alternatively, the network may instructthe WTRU 100 to use an absolute power, but provides a rule when the WTRU100 is allowed to provide feedback. For example, the WTRU 100 may beallowed to send the feedback if the received reference channel power isbelow a pre-defined value. Alternatively, the conventional HS-SCCH maybe modified to include power control information related to thetransmission of feedback over the shared feedback channel. Power offsetor relative power command, (e.g., increase or decrease), bits may beintroduced in the HS-SCCH to adjust the transmission power of the WTRUover the shared feedback channel. Optionally, the WTRU 100 may includechannel quality information in the feedback.

A scheme for sending a CQI via the P-RAFCH is disclosed hereinafter. ACQI is also transmitted via the P-RAFCH. While the CQI feedback may beeither scheduled or triggered, the Node-B must be able to differentiatebetween NACK only feedback, CQI only feedback, and CQI feedback which istriggered by a NACK, (i.e., NACK+CQI). The P-RAFCH burst includes a datatype indicator for indicating NACK only, CQI only or NACK+CQI, a datafield for carrying CQI bits if needed, and a reference field forcarrying a modulation phase and power reference, if needed.

These fields may be mapped into the burst by time division multiplexing(TDM), (i.e., each data is transmitted in its own time segment).Alternatively, the fields may be mapped into the burst by code divisionmultiplexing (CDM), (e.g., a signature based structure as in the PRACHpreamble). Alternatively, the fields may be mapped into the burst byfrequency division multiplexing (FDM). FDM is particularly appropriatefor systems, such as long term evolution (LTE), where a number ofsub-carriers may be utilized. The basic physical channel resources forcarrying these fields may be, but not necessarily, orthogonal at leastat the WTRU.

The data field, if present, may use any multi-dimensional modulationschemes with each physical channel resource (time slot, signature,carrier, etc.) providing a dimension in the modulation vector space.Some examples of possible modulation schemes are as follows:

(1) Multi-dimensional m-phase shift keying (PSK) (including binary phaseshift keying (BPSK) (m=2), quadrature phase shift keying (QPSK) (m=4)),m is an integer power of 2. The number of physical channel resourcesrequired is M/log₂ m, and additional phase and power reference isrequired.

(2) Multi-dimensional m-quadrature amplitude modulation (QAM) (includingBPSK (m=2) and QPSK (m=4)), m is an integer power of 2. The number ofphysical channel resources required is M/log₂ ^(m), and additional phaseand power reference is required.

(3) m-ary orthogonal modulation. The number of physical channelresources required is M (i.e., m=M), and additional phase and powerreference is not needed.

(4) m-ary bi-orthogonal modulation. The number of physical channelresources required is M/2 (i.e., m=M/2), and additional phase and powerreference is required.

(5) Multi-dimensional on-off keying, (i.e., M/2 carriers are either withor without power). The number of physical channel resources required isM/2, (i.e., m=M/2), and additional phase and power reference is notrequired.

The modulation scheme to be used should be signaled to the WTRU. Certainmodulation schemes may require the use of a phase and power reference,while others do not. The reference, if required, may be sent togetherwith the data type indicator. The data type indicator and the referencefield may be sent on separate physical resources. Alternatively, onlythe data type indicator is sent and the reference field is derived fromit using decisions feedback, (i.e., the data type indicator is assumedto be demodulated correctly, which permits its re-use as a referencesignal).

Additionally, in order to avoid the explicit transmission of the datatype indicator, a CQI may always be triggered by the need to transmit aNACK, (i.e., a NACK and a CQI are always sent together). Alternatively,if a NACK is sent and a CQI does not need to be sent, the data fieldcorresponding to the highest CQI value may be used. These types oftransmissions are referred to as an implicit data type format. The useof this format should be signaled to the WTRU.

The Node-B detects the presence of power over the complete burst. Ifpower is detected in a burst space, and a data type indicator is used,the Node-B reads the data type indicator. If a CQI is present, the CQIis demodulated according to the modulation scheme used. If the implicitdata type format is used, the presence of power indicates a NACK and aCQI transmission.

Because of the multicast nature of the transmissions and the need toserve most or all WTRUs, the Node-B may collect CQIs over some timeperiod. The Node-B selects the minimum CQI over this time period andschedules data rates according to the minimum CQI. In this manner allWTRUs may be highly likely to be served.

This scheme does, however, have a disadvantage that a WTRU with a badchannel condition may significantly reduce the throughput of the wholesystem. The Node-B has no way to identify that such a WTRU exists in adirect way because all feedback from the WTRUs is anonymous. In order tosolve this problem, the Node-B may collect statistics about CQItransmissions and may ignore any CQIs that are statistically very farfrom the majority. The Node-B may then select a minimum CQI from theremaining CQIs and uses that as a baseline.

Alternatively, the Node-B may select a certain small subset, (e.g.,lower 20% or lower 10%), of CQIs after the removal of outliers. TheNode-B may then use an average of these, (e.g. the actual average, amedian, etc.). Because of the multicast nature, the highest CQIs areunlikely to have any impact on the system operation. Thus, the WTRU maynot send the highest possible CQI value.

Another embodiment of layer 2/3 (L2/3) based operation is disclosedhereinafter. A WTRU 100 listens to network signaling which tells theWTRU 100 when, how often, and to whom to report feedback to the downlinkshared service. The WTRU 100 decodes signals on an allocated TTI for ashared downlink service. The WTRU 100 then collects statistics ofdecoding success or failure rate and compares to the decoding statisticsto a pre-defined threshold that is provided by the network. The WTRU 100sends feedback if the decoding statistics is worse than the pre-definedthreshold.

If the WTRU 100 has selected a Node-B out of several synchronizedNode-Bs which transmit the same data, the WTRU 100 transmits thefeedback to that selected Node-B only. If the WTRU 100 performs softcombining of signals from a plurality of Node-Bs in an active set, theWTRU 100 sends the feedback to the strongest Node-B in the active set.

The network may indicate desired transmission of the NACK on a celldifferent from the one where the downlink shared service, (e.g., MBMS),is received. The cell is indicated by the network.

The downlink shared service may be mapped to a code that will betransmitted with the NACK. Alternatively WTRU connection ID may besignaled. Alternatively, if using a PRACH for the feedback, the physicalchannel access opportunities may be mapped to the downlink sharedservice. The mapping may be indicated by the network. If needed, CQIinformation may be transmitted together with the NACK or in its place.Since the signaling is at L2/3, a larger number of bits are supported ina straightforward fashion.

Some downlink shared services, (e.g., video), may use a layered QoSmechanism where certain users get higher throughput and quality thanothers. In a wireless system, an important factor that determines theQoS of a user is the throughput achievable given the location of theuser in the system. The maximum throughput achievable at cell edge istypically less than the one achievable around the cell center. Thelayered QoS may be supported without feedback from dedicated physicalchannels.

One conventional layered QoS mechanism, (e.g., digital videobroadcasting (DVB)), is based on hierarchical modulation. Inhierarchical modulation, multiple data streams, (typically ahigh-priority and a low-priority), are modulated into a single signalthat is received by all users. Users with good signal quality may decodeboth data streams while users with low signal quality may decode onlythe high-priority stream. For instance, the streams may be encoded as a16 quadrature amplitude modulation (16QAM) signal. The quadrant wherethe signal is located represents two high priority bits whereas theposition of the signal within the quadrant represents two low prioritybits. Users with good signal quality are able to decode the signal as16QAM while users with low signal quality can only decode the signal asquadrature phase shift keying (QPSK) and extract only the high prioritybits.

In accordance with the embodiments, some new signaling is provided. Fromthe network point of view, it would be unsatisfactory that all WTRUsreport their ACK or NACK feedback based on decoding of the high prioritystream only because it would lack information about the performance offavorably located WTRUs. On the other hand, having all WTRUs providingfeedback based on decoding of all streams is also unsatisfactory becausenon-favorably located WTRUs would overload the P-RAFCH with NACK.

The network sets at least one CQI threshold to determine on which streameach WTRU should provide feedback. The CQI threshold(s) is signaled fromthe network, (e.g., on the BCCH, dedicated control channel (DCCH), orMBMS control channel (MCCH) for broadcast, multicast, or unicast).

A WTRU 100 measures its own CQI (or average CQI). The WTRU 100 comparesthe measured CQI to the CQI threshold(s) and determines the smallest CQIthreshold higher than the measured CQI. This CQI threshold correspondsto a certain subset of stream(s) that the WTRU 100 needs to reportfeedback. The WTRU 100 reports ACK or NACK feedback on the decoding ofthe subset of stream(s) determined based on the CQI comparison. It ispossible to further restrict the subset of streams to report feedbackbased on WTRU subscription to the high quality service.

A particular CQI threshold may be set below which the WTRU 100 is notallowed to provide feedback. For example, in the case that there areonly two streams, (high priority stream and low priority stream), andtwo CQI thresholds, (high CQI threshold and low CQI threshold), are set,if the measured CQI is above the high CQI threshold, the WTRU 100 mayreport feedback on both high priority and low priority streams. If themeasured CQI is below the high CQI threshold but above the low CQIthreshold, the WTRU 100 may report feedback on the high priority streamonly. If the measured CQI is below the low CQI threshold, the WTRU 100may not provide feedback at all.

The Node-B 120 may change the CQI threshold(s) from time to time basedon load conditions. For instance, in case the load of the Node-B 120 dueto other services is low, the Node-B 120 may allocate more resources tothe downlink shared services and employ less aggressive MCS to encodethe streams, which allows more WTRUs to enjoy high QoS. In case of highcontention between the downlink shared services and other services, theNode-B 120 may use more aggressive MCS to transmit the streams therebyreducing the amount of resources for the downlink shared services.

Alternatively, the multiple streams may be transmitted separately indifferent time or using different codes. For instance, a high prioritystream may be transmitted with a less aggressive MCS while a lowpriority stream may be transmitted with a more aggressive MCS. Thisallows more flexibility in the selection of the MCS and CQI thresholdsfor decoding the streams. The disadvantage is that it is less efficientsince the streams are not combined in the same signal.

It should be noted that although the feedback mechanism above isdescribed in terms of CDMA systems, it is generic and may be applied toany wireless communication systems, and the physical channel, P-RAFCH,may be defined by a combination of any physical resources.

A method of counting the number of WTRUs which are listening to aparticular Node-B transmission by using a contentions feedback channel,(such as P-RAFCH), will be explained hereafter. Suppose that there are anumber (M) of WTRUs satisfying a configured criterion. The number ofthese WTRUs is counted by forcing each one of these WTRUs to send asignal, (e.g., ACK, NACK, PING, or the like), on the P-RAFCH.

In accordance with one embodiment, a particular physical resource, (suchas subcarrier, code, timeslot, spatial stream, or combination of these),may be allocated for each WTRU and the number of physical resource thatare actually used may be counted. This will generate a very accurateoutcome ignoring communication error. However, it may be inefficient interms of overhead because if there are a lot of WTRUs that may bepresent, a lot of physical resources are needed.

Alternatively, N physical resources may be reserved for the P-RAFCH thatthe WTRUs may access at random. The number of physical resourcesactually used is then counted, and the number of WTRUs (M) is estimatedbased on the number of used physical resources. While this estimate maynot be precise, the error may be tolerable for many applications. Thecounting error depends on the number of available physical resources (N)and the number of WTRUs (M). The number of N physical resources that isneeded for acceptable error may be obtained by solving followingequation (4) for N:

$\begin{matrix}{M_{\max} = {\frac{c}{p}N\; {\ln (N)}}} & {{Equation}\mspace{14mu} (4)}\end{matrix}$

where M_(max) is the number of WTRUs that may be present, c>1 is atolerance factor, (e.g., c=2), and p is the probability with which aWTRU transmits on the P-RAFCH if the conditions for transmission aremet, which will be described in detail below. For large M_(max), N maybe significantly lower than M_(max) resulting in substantial reductionin signaling overhead in the uplink. Any other number of physicalresources may be used depending on the acceptable error.

A P-RAFCH is a physical channel that is defined by allocating one ormore physical resources, (e.g., sub-carriers, codes, time-slots, spatialstreams, or combination of all or some of these). N physical resourcesare reserved for the P-RAFCH for every predefined time interval. Thispredefined time interval is referred to as a P-RAFCH frame. The P-RAFCHframe may correspond to a frame, super-frame, sub-frame, slot, etc. indifferent wireless communication standards. More than one P-RAFCH may bedefined in a cell.

A “transmission criterion” (TC) is defined for each P-RAFCH. A TC for aP-RAFCH may be at least one of the following, but not limited to:

(1) Successful reception of a data packet or a block of data on aparticular downlink physical channel;

(2) Successful reception of a block of data on a particular data service(which may be spread across multiple channels);

(3) Reception of a particular signaling command;

(4) Occurrence of a measurement event; or

(5) Failure to receive a particular transmission after a specifiednumber of times.

A TC needs to yield a YES/NO answer, and each WTRU must be able todetermine it independently without any external coordination.

FIG. 6 is a flow diagram of an example process 600 of evaluating atransmission criterion for transmitting a P-RAFCH. In each P-RAFCH framefor each P-RAFCH, a WTRU determines whether a TC associated with theP-RAFCH is satisfied (step 602). The TC associated with each P-RAFCH isprovided as a part of P-RAFCH setup. If the TC has not been satisfied,the process 600 ends, (i.e., the WTRU does not transmit a P-RAFCH inthis P-RAFCH frame). If the TC has been satisfied, the WTRU mayoptionally make a decision whether to send a P-RAFCH or not based on apreconfigured probability (p) of sending a P-RAFCH (step 604). Theprobability (p) may be set to ‘1’ so that the WTRU may always send thesignal once the TC is satisfied. If the WTRU decides not to send aP-RAFCH at step 606, the process 600 ends, (i.e., the WTRU does nottransmit a P-RAFCH in this P-RAFCH frame). If the WTRU decides to send aP-RAFCH at step 606, the WTRU randomly selects one of N availableP-RAFCH physical resources associated with the P-RAFCH (step 608). TheWTRU then transmits a pre-defined signal using the selected physicalresource (step 610). All WTRUs may transmit the same signal and thesignal may be designed in such a way that collisions are unlikely toresult in nulling of the signal, (e.g., a constant amplitude and phase).

In each P-RAFCH frame and for each P-RAFCH the Node-B estimates whethereach physical resource was used, (e.g., using a signal detectionscheme). The Node-B counts the number of used physical resources andestimates the number of WTRUs (M) accessed the P-RAFCH based on thenumber of used physical resources.

Many services and operational improvements are possible by using thenumber of WTRUs (counted or estimated). In some applications, abroadcast service transmits certain contents to users. The broadcastermay need to know how many users are listening to the channel, forexample in order to enable the broadcaster to estimate how much tocharge advertisers whose contents are broadcast on the same channel. Inthis case, it is not important to know who those listeners are, but onlyhow many of them. To this effect the listeners are instructed to send asignal (PING).

In some applications of broadcast services, the network may wish to makesure that a service is available to at least a certain number orpercentage of WTRUs in the cell. To ensure this, it needs to estimatethe total number of WTRUs attempting to receive the service and how manyof these are receiving it successfully. To do so, any two of thefollowing three quantities are needed: the number of successfulreceptions (ACKs), the number of failures (NACKs), and the number ofWTRUs present (PINGs). This may be accomplished by defining two P-RAFCHsfor the service, (for example, one for ACKs and one for NACKs, oralternatively one for PINGs and one for ACKs or NACKs). Using a totalcount (PINGs) may be preferable as this quantity is likely to remainstable for a prolonged period of time and such a count may be requestedperiodically using a more general P-RAFCH for feedback.

In both broadcast and unicast unacknowledged services, (i.e., theservices without dedicated feedback), the Node-B may wish to utilizeseveral retransmissions to ensure proper data delivery. On the otherhand, the Node-B may want to fine-tune the number of retransmissions tominimize the number of retransmissions while delivering appropriatequality of service (QoS) to at least some minimal number of WTRUs. TheP-RAFCH may be used for this object by defining TC to be lack ofsuccessful decoding after a predetermined number of retransmissions. AWTRU attempts to decode data after every re-transmission, and if theWTRU fails after a predefined number of attempts, the WTRU sends a NACKon the P-RAFCH. By counting the NACKs and estimating the number of WTRUswhich responded, the Node-B may appropriately select the number ofretransmissions which minimizes air interface usage while meeting therequired QoS. This mechanism may be used for adjusting power control forthese types of services.

The method for estimating the number of WTRUs (M) based on the number ofobserved used physical resources out of a total of N physical resourcesin a P-RAFCH frame is explained in detail. It should be noted that thisis not the only estimator that may be used, although the estimator Mprovides fairly good performance especially when M is likely to be quitelarge.

It is assumed that p=1, (i.e., a WTRU always transmits on a P-RAFCH if aTC is satisfied). It should be noted that setting p=1 is an example andp may be set differently. When p is not equal to ‘1’, equation (8) belowneeds to be multiplied by a factor of 1/p. Because the generation ofSEND/NO SEND decision for each WTRU is independent of other events, theanalysis extends to other value of p: 0<p<1, by simply multiplying theestimator by a factor of 1/p.

Let T be the number of used physical resources in a P-RAFCH frame with atotal of N physical resources. T is a random variable, 0≦T≦N. Based onthis, the number of WTRUs that actually sent feedback is estimated,(i.e., count the WTRUs which sent an ACK).

The distribution of T given M is a distribution that when M agentspicked one out of N≧1 objects (with replacement). Only T distinctobjects are actually picked. The problem is closely related to thecoupon collector problem, which is a standard combinatorial problem. Thedistribution is given as follows:

$\begin{matrix}\begin{matrix}{{{\Pr \left\{ {T = t} \right\}} = {\frac{N!}{\left( {N - t} \right)!}\frac{S\left( {M,t} \right)}{N^{M}}}},} & {{0 \leq t \leq {\min \left( {N,M} \right)}},} \\{{\Pr \left\{ {T = t} \right\}} = 0} & {{otherwise},}\end{matrix} & {{Equation}\mspace{14mu} (5)}\end{matrix}$

where S(M,T) is the Stirling number of the second kind:

$\begin{matrix}{{S\left( {M,t} \right)} = {\frac{1}{t!}{\sum\limits_{j = 0}^{t}{\left( {- 1} \right)^{j}\begin{pmatrix}t \\j\end{pmatrix}{\left( {t - j} \right)^{M}.}}}}} & {{Equation}\mspace{14mu} (6)}\end{matrix}$

The distribution is quite complex. In particular, the maximum likelihood(ML) estimate is difficult to obtain as maximizing Equation (4) over Mis difficult analytically or computationally. It is well known thatasymptotically:

$\begin{matrix}{{E\lbrack T\rbrack} = {{N\left( {1 - \left( {1 - \frac{1}{N}} \right)^{M}} \right)} \approx {{N\left( {1 - ^{{- M}/N}} \right)}.}}} & {{Equation}\mspace{14mu} (7)}\end{matrix}$

While equation (7) is accurate only asymptotically, it is good enough.From equation (7), the following approximate estimator may be used:

$\begin{matrix}{{\overset{\Cap}{M}(t)} = {\frac{\ln \left( {1 - \frac{t}{N}} \right)}{\ln \left( {1 - \frac{t}{N}} \right)} \approx {{- N}\; {{\ln \left( {1 - \frac{t}{N}} \right)}.}}}} & {{Equation}\mspace{14mu} (8)}\end{matrix}$

The approximate estimator may be used instead of the exact one to savesome complexity, if needed. It can be shown that equation (8) is theminimum variance unbiased estimator of M based on T.

If t=N, the estimate {circumflex over (M)}(N)=∞. This makes intuitivesense in the view of an ML estimation, i.e., maximizing a posteriorilikelihood. In case that all physical resources in the P-RAFCH framehave been used, the number of WTRUs that makes it most likely that thiswould happen should be infinite, absent any upper bound. Using thisintuition, a design criterion is suggested for selecting an appropriatenumber of feedback slots, given a maximum expected number of WTRUs.Specifically,

$\begin{matrix}{{{M_{\max}(N)} = {{{- {cN}}\; {\ln \left( {1 - \frac{N - 1}{N}} \right)}} = {{cN}\; {\ln (N)}}}},} & {{Equation}\mspace{14mu} (9)}\end{matrix}$

which can be solved for N numerically given M_(max). C is anappropriately selected constant, which may even be set larger than 1.For example, c=2 would be a reasonable choice.

Although the features and elements are described in the preferredembodiments in particular combinations, each feature or element can beused alone without the other features and elements of the preferredembodiments or in various combinations with or without other featuresand elements. The methods or flow charts provided in the presentinvention may be implemented in a computer program, software, orfirmware tangibly embodied in a computer-readable storage medium forexecution by a general purpose computer or a processor. Examples ofcomputer-readable storage mediums include a read only memory (ROM), arandom access memory (RAM), a register, cache memory, semiconductormemory devices, magnetic media such as internal hard disks and removabledisks, magneto-optical media, and optical media such as CD-ROM disks,and digital versatile disks (DVDs).

Suitable processors include, by way of example, a general purposeprocessor, a special purpose processor, a conventional processor, adigital signal processor (DSP), a plurality of microprocessors, one ormore microprocessors in association with a DSP core, a controller, amicrocontroller, Application Specific Integrated Circuits (ASICs), FieldProgrammable Gate Arrays (FPGAs) circuits, any other type of integratedcircuit (IC), and/or a state machine.

A processor in association with software may be used to implement aradio frequency transceiver for use in a wireless transmit receive unit(WTRU), user equipment (UE), terminal, base station, radio networkcontroller (RNC), or any host computer. The WTRU may be used inconjunction with modules, implemented in hardware and/or software, suchas a camera, a video camera module, a videophone, a speakerphone, avibration device, a speaker, a microphone, a television transceiver, ahands free headset, a keyboard, a Bluetooth® module, a frequencymodulated (FM) radio unit, a liquid crystal display (LCD) display unit,an organic light-emitting diode (OLED) display unit, a digital musicplayer, a media player, a video game player module, an Internet browser,and/or any wireless local area network (WLAN) module.

1. A method implemented in a wireless transmit/receive unit (WTRU) forsending a signal on a contentious feedback channel for a downlink sharedservice provided to a plurality of WTRUs, the method comprising:determining whether a transmission criterion associated with a physicalrandom access feedback channel (P-RAFCH) is satisfied, the P-RAFCH beinga contentious channel shared by a plurality of WTRUs for sendingfeedback for the downlink shared service; selecting a physical resourcerandomly among a plurality of physical resources assigned for theP-RAFCH on a condition that the transmission criterion is satisfied; andsending a pre-configured signal using the selected physical resource. 2.The method of claim 1 wherein the pre-configured signal is sent onlyonce without requiring an acknowledgement.
 3. The method of claim 1wherein the transmission criterion is at least one of successfulreception of a data packet on a downlink physical channel, successfulreception of a data packet on a data service, reception of a signalingcommand, occurrence of a measurement event, or failure to receive atransmission after a specified number of times.
 4. The method of claim 1further comprising: determining whether to send the pre-configuredsignal based on a pre-configured probability of sending thepre-configured signal, wherein the pre-configured signal is sent basedon the determination.
 5. A method for estimating a number of wirelesstransmit/receive units (WTRUs) satisfying a transmission criterion, themethod comprising: receiving a pre-configured signal on a physicalrandom access feedback channel (P-RAFCH) from a plurality of WTRUs, theP-RAFCH being a contentious channel used for sending feedback when atransmission criterion associated with the P-RAFCH is satisfied; andcalculating a number of the WTRUs based on a number of physicalresources used among a plurality of physical resources reserved for theP-RAFCH.
 6. The method of claim 5 wherein the number of WTRUs iscalculated considering a probability that a WTRU transmits thepreconfigured signal on a condition that the transmission criterion issatisfied.
 7. The method of claim 5 wherein a particular physicalresource is assigned to each of the WTRUs and the number of WTRUs iscalculated by counting the number of physical resources used.
 8. Themethod of claim 5 wherein a per-determined number of physical resourcesis assigned for the P-RAFCH, and the number of WTRUs is estimated basedon the number of used physical resources.
 9. The method of claim 8wherein the number of WTRUs is estimated by −N ln(1−t/N), N being anumber of physical resources assigned for the P-RAFCH, and t being anumber of actually used physical resources.
 10. The method of claim 5wherein the transmission criterion is at least one of successfulreception of a data packet on a downlink physical channel, successfulreception of a data block on a data service, reception of a signalingcommand, occurrence of a measurement event, or failure to receive atransmission after a specified number of times.
 11. A wirelesstransmit/receive unit (WTRU) configured to send a signal on acontentious feedback channel for a downlink shared service, the WTRUcomprising: a controller configured to determine whether a transmissioncriterion associated with a physical random access feedback channel(P-RAFCH) is satisfied, and selecting a physical resource randomly amonga plurality of physical resources assigned for the P-RAFCH on acondition that the transmission criterion is satisfied, the P-RAFCHbeing a contentious channel shared by a plurality of WTRUs for sendingfeedback for the downlink shared service; and a transmitter configuredto send a pre-configured signal using the selected physical resource.12. The WTRU of claim 11 wherein the pre-configured signal is sent onlyonce without requiring an acknowledgement.
 13. The WTRU of claim 11wherein the transmission criterion is at least one of successfulreception of a data packet on a downlink physical channel, successfulreception of a data packet on a data service, reception of a signalingcommand, occurrence of a measurement event, or failure to receive atransmission after a specified number of times.
 14. The WTRU of claim 11wherein the controller is configured to determine whether to send thepre-configured signal based on a pre-configured probability of sendingthe pre-configured signal and send the pre-configured signal based onthe determination.
 15. A Node-B configured to estimate a number ofwireless transmit/receive units (WTRUs) satisfying a transmissioncriterion, the Node-B comprising: a receiver configured to receive apre-configured signal on a physical random access feedback channel(P-RAFCH) from a plurality of WTRUs, the P-RAFCH being a contentiouschannel used for sending feedback when a transmission criterionassociated with the P-RAFCH is satisfied; and a controller configured tocalculate a number of the WTRUs based on a number of physical resourcesused among a plurality of physical resources reserved for the P-RAFCH.16. The Node-B of claim 15 wherein the controller is configured tocalculate the number of WTRUs considering a probability that a WTRUtransmits the preconfigured signal on a condition that the transmissioncriterion is satisfied.
 17. The Node-B of claim 15 wherein a particularphysical resource is assigned to each of the WTRUs and the controller isconfigured to calculate the number of WTRUs by counting the number ofphysical resources used.
 18. The Node-B of claim 15 wherein aper-determined number of physical resources is assigned for the P-RAFCH,and the controller is configured to estimate the number of WTRUs basedon the number of used physical resources.
 19. The Node-B of claim 18wherein the controller is configured to estimate the number of WTRUs by−N ln(1−t/N), N being a number of physical resources assigned for theP-RAFCH, and t being a number of actually used physical resources. 20.The Node-B of claim 15 wherein the transmission criterion is at leastone of successful reception of a data packet on a downlink physicalchannel, successful reception of a data block on a data service,reception of a signaling command, occurrence of a measurement event, orfailure to receive a transmission after a specified number of times.